Rechargeable batteries, i.e., secondary batteries, have become increasingly important in recent years as vehicle-mounted power sources and as power sources for personal computers and portable and mobile electronic devices. In particular, lithium secondary batteries (typically lithium ion secondary batteries), which are light and provide high energy densities, are expected to be favorably used as vehicle-mounted large-scale power sources. In lithium secondary batteries of this type, charge and discharge occur by the shuttling of the lithium (Li) ion back and forth between a positive electrode and a negative electrode that are structural components of the battery, and the positive electrode material (positive electrode active material) is the most important component for determining the battery performance. For example, the discharge capacity is established by how much Li ion can be inserted into and extracted from the positive electrode active material, and the resulting battery voltage varies as a function of the materials used and the crystal structure.
Attention is currently being focused on olivine-type LiMPO4 (M: Fe, Ni, Mn, Co, and so forth), which has a crystal structure in the Pnma space group, for the positive electrode material of lithium secondary batteries. Within this series, LiNiPO4 is expected to be a high-potential positive electrode material because it exhibits a high potential of 5 V or more. Patent Literature 1 and Patent Literature 2 are examples of the prior art for this type of olivine LiNiPO4.